The present invention relates to a single mode optical fiber widely used in an optical communication system, and particularly relates to optical coupling between optical fibers, optical coupling between an optical fiber and an optical device, and so on.
Increase in capacity of an optical fiber communication network has been demanded intensively with the rapid advance of popularization of Internet in recent years. Development of wavelength division multiplexing (WDM) of optical communication has progressed rapidly as means for increasing the capacity. As for basic constituent members of an optical communication system, a semiconductor laser is required as a light source, a photodiode is required as a photo detector, and an optical fiber, an optical amplifier or the like is required as an optical transfer medium. In addition, in WDM optical communication, optical devices with good wavelength selectability such as an optical demultiplexer, a filter and an isolator are required because wavelengths of light slightly different from each other can transmit information individually.
To construct an optical fiber communication network, these constituent members need to be coupled to one another optically with a small loss. It is therefore very important to use an optical coupling technique including various kinds of optical coupling such as coupling between a semiconductor laser or photo diode and an optical fiber, coupling in the case where an optical device (a filter or an isolator) is inserted in between optical fibers, coupling between optical fibers, coupling between an optical fiber and an optical waveguide, and so on.
Such optical coupling has been heretofore generally performed by joining optical fibers to each other directly or through a lens. In the case of joining of optical fibers to each other directly, loss is not so high, but permissible tolerance of alignment is very strict to thereby result in increase in assembling cost. In the case of using a lens, a gradient index rod lens is used for various kinds of application because the lens is shaped cylindrically and matches the optical fibers geometrically so as to be suitable for direct insertion into a sleeve shaped like a hollow cylinder, for arrangement in a V groove, and so on (for example, JP-A-60-91316).
The refractive-index distribution of the gradient index rod lens can be given by the expression:n(r)2=n02·{1−(g·r)2+h4(g·r)4+h6(g·r)6+h8(g·r)8+ . . . }in which r is a distance from the optical axis, n(r) is a refractive index in a position at the distance r from the optical axis, n0 is a refractive index on the optical axis, r0 is the radius of the rod lens, g is a refractive-index distribution coefficient, and h4, h6, h8 . . . are refractive-index distribution coefficients respectively.
The lens function of the gradient index rod lens varies according to the lens length Z.
The following basic usage can be made on the basis of the periodic length P defined by the expression:P=2π/g    (1) The case of Z=0.25P: Light of a light source disposed on an end surface is collimated.    (2) The case of Z=0.50P: Light of a light source disposed on an end surface is focussed on an opposite end surface.
The rod lens, however, generally has a diameter of about 1 mm which is considerably different from 125 μm that is the outer diameter of a standard single mode optical fiber. For this reason, it is necessary to prepare an exclusive-use holder for holding both the rod lens and the optical fiber or to hold an end portion of the optical fiber by use of a capillary having a diameter equal to that of the rod lens. Hence, the number of parts increases and assembling becomes complex. Moreover, the resulting system occupies a large volume as a whole. As a result, this causes increase in cost.
As means for solving these problems, there has been an attempt that a lens having a diameter equal to that of an optical fiber is disposed at a forward end of the optical fiber. For example, in Journal of Lightwave Technology, Vol.17, No.5, P.924, 1999, there has been proposed a structure in which a gradient index optical fiber (GIF) of quartz glass having a diameter equal to that of a single mode optical fiber (SMF) is fused to a forward end of the SMF through a homogeneous quartz glass spacer. In such a structure, these optical fibers can be arranged in a V groove or the like easily, so that great reduction in production cost can be expected.
In the aforementioned example, however, the refractive-index distribution cannot be controlled sufficiently because an existing quartz glass multi-mode optical fiber for communication is used. Hence, performance sufficient to be used for the lens function cannot be brought out yet. Moreover, the lens function of the GIF for communication is concentrated into the core portion near the optical axis. For this reason, refracting power (the value of the refractive-index distribution coefficient q in the aforementioned expression giving the refractive-index distribution) is so large that the collimated luminous flux becomes thin.
In order to thicken the collimated luminous flux, it is necessary to separate end surfaces of these two optical fibers by a specific distance. In the aforementioned example, a spacer is interposed to define this distance. For this reason, the number of parts does not change compared with the case of use of a holder or a capillary though there is an advantage in that the outer diameters of the two optical fibers can be made coincident with each other. Hence, there is still a problem that the original purpose cannot be achieved.